The present invention relates to rotating electrical machines and more particularly to permanent magnet enhanced rotors for generators of automobiles.
Alternators (generators) are used to provide electrical power for running accessories and to charge batteries of automobiles. The combination of increased electrical power demand and generally smaller vehicles creates the demand for greater power output and higher efficiency without greatly increasing the size of the alternator.
In an attempt to meet all of these requirements, several known designs employ a xe2x80x9chybridxe2x80x9d alternator design in which a traditional wound-field rotor is supplemented with permanent magnets. The addition of permanent magnets increases the rotor flux, and hence power output, as compared with traditional alternators. One such design is described in U.S. Pat. No. 4,959,577 (hereinafter the ""577 patent), issued to Radomski. In this patent, permanent magnets are installed between the pole fingers of an alternator. The magnets are magnetized in a tangential direction with respect to the rotor""s axis of rotation. While the design of the ""577 patent can provide some advantages over standard Lundell rotors, the tangential magnetization is believed to be inefficient because a substantial amount of the permanent magnet flux is not directly linked with the stator windings. Such linking may provide benefits of higher electrical output and higher power density (i.e. power output per unit volume of the alternator) of the alternator.
In an attempt to directly link permanent magnet flux with the stator windings, known designs have altered the arrangement of the permanent magnets. One such design is disclosed in U.S. Pat. No. 5,965,967 (hereinafter the ""967 patent), issued to Liang.
In the ""967 patent, a rotor includes a field coil magnetically coupled with a first pole piece and a second pole piece which when energized magnetizes the fingers of the first pole piece and second pole piece with opposite magnetic polarities. The rotor also comprises a plurality of permanent magnets, at least two disposed between each first pole finger and an adjacent second pole finger and having a radially-outward surface and an adjacent tangentially-facing surface of first electromagnetic polarity and having a radially-inward surface and an adjacent tangentially surface of an opposite magnetic polarity.
Although the design of the ""967 patent directly links permanent magnetic flux with the stator windings, and hence increases power output, the permanent magnet arrangement may not make optimal use of the permanent magnets. The ""967 patent has two permanent magnets per rotor slot having opposite polarities. This arrangement introduces harmonic flux, which results in decreased efficiency and increased magnetic noise.
In addition, the design of the ""967 patent may not be used in salient pole type synchronous machines, whose rotor poles are not tapered, because the net permanent magnet flux linking to the stator winding is zero. The net flux linkage is zero because the permanent magnet flux linkage produced by the permanent magnets (preferably 2 magnets per rotor slot) in the same rotor slot cancels each other out.
It would therefore be desirable to increase the power output of a hybrid permanent magnet machine while increasing the efficiency and decreasing the magnetic noise compared to known systems.
One object of the present invention is to increase the power output of hybrid permanent magnet synchronous machines. A second object of the present invention is to improve the efficiency of the hybrid permanent magnet synchronous machines. A third object of the present invention is to lower magnetic noise of a hybrid permanent magnet synchronous machine.
The present invention accomplishes all three objects by providing a new arrangement of permanent magnets that can be used in either a Lundell-type or salient pole-type rotor.
The present invention is a rotor for use in an electrical machine, where the rotor has an axis of rotation and has a first and second pole piece, a field winding (preferably a field coil or rotor coil) on the rotor, and a set of permanent magnets disposed between the first and second pole pieces. The first pole piece and second pole piece each has a plurality of axially-extending pole fingers. The field winding is magnetically coupled with the first pole piece and second pole piece which when energized magnetizes the first pole fingers with a north magnetic polarity and the second pole fingers with a south magnetic polarity.
The permanent magnets are preferably trapezoidal shaped and are grouped into two sets. One of the first set of permanent magnets is disposed in every other rotor slot with its radially-inward surface adjacent to the inner rotor portion and its side surfaces abutting adjacent pole pieces. One of the second set of permanent magnets is disposed within each of the other rotor slots not occupied by the first set of permanent magnets with its radially-inward surface adjacent to the inner rotor portion and its side surfaces abutting adjacent pole pieces. Preferably, the length of each permanent magnet substantially extends the length of the rotor slot and the length of the adjacent pole pieces that it is contained within. The first set of permanent magnets are magnetized with its radially-outward surface having a north magnetic polarity and each side surface having the magnetic polarity of its adjacent pole pieces. The second set of permanent magnets are magnetized with its radially-outward surface having a south magnetic polarity and each side surface having the magnetic polarity of its adjacent pole piece. Each side surface of each permanent magnet preferably contacts each corresponding adjacent pole piece.
As a result of this configuration of permanent magnets, the present invention increases the rotor flux in two ways. First, permanent magnet flux travels from one set of permanent magnets to the second set of permanent magnets through the air gap, thereby increasing the flux linkage of the stator winding which in turn increases the alternator power output. Second, permanent magnet flux in the rotor cancels flux generated by the field current to some extent, which reduces rotor core saturation and further increases rotor flux. Therefore, the new permanent magnet arrangement has better permanent magnet utilization, which leads to higher electrical outputs and hence better performance.
Further, this new permanent magnet arrangement has lower harmonic fluxes than known arrangements such as the ""967 patent because the permanent magnet flux changes more gradually in space. As a result, the loss caused by harmonic flux is lower, which gives rise to higher efficiency.
Also, one embodiment of this new permanent magnet arrangement has less magnetic noise than previous arrangements because the permanent magnets are not in parallel with the stator slots, allowing flux in the stator teeth to vary gradually rather than abruptly and hence reducing magnetic noise.
Other objects and advantages of the present invention will become apparent upon considering the following detailed description and appended claims, and upon reference to the accompanying drawings.